Alkaline battery separator and process for producing the same

ABSTRACT

An alkaline battery separator which enables the preparation of a battery with a good yield and workability is provided. The alkaline battery separator of the present invention comprises a fiber sheet mainly comprising hydrophilicity-imparted polyolefin fibers having a fiber diameter of 8 μm or more, and a part of the hydrophilicity-imparted polyolefin fibers is composed of high-strength fibers having a tensile strength of 5 g/d or more.

TECHNICAL FIELD

The present invention relates to an alkaline battery separator and aprocess for producing the same.

BACKGROUND ART

In an alkaline battery, a separator is used to separate a positiveelectrode and a negative electrode from each other to prevent a shortcircuit therebetween, and further, to hold an electrolyte thereon andprovide a smooth electromotive reaction.

Recently, in electronic equipment, the space allotted for the alkalinebattery has been reduced due to the need for miniaturization andweight-saving. Nevertheless, the performance requirement for such asmaller alkaline battery is the same as or higher than that for aconventional battery, and therefore, it is necessary to increase thecapacity of the battery, and also increase the amounts of activematerials in the electrodes. Thus, the volume allotted in the batteryfor the separator must be reduced, the thickness of the separator mustbe reduced thinner , and electrodes and separators must be placed inclose contact to prevent the formation of a space therebetween when anelectrodes-group is produced. Nevertheless, when thinner separators areused, and a strong tension is applied to ensure the close contactbetween the electrodes and separators, any flash occurring at theelectrode can puncture the separator and cause a short circuit, an edgeof the electrode might then tear the separator, and thus lower theyield.

Japanese Unexamined Patent Publication (Kokai) No. 7-29561 or JapaneseUnexamined Patent Publication (Kokai) No. 8-138645, for example,disclose that fine fibers having a small diameter, for example,approximately 5 μm, are used to make the separator thinner.Nevertheless, the separator obtained from such fine fibers lacksstiffness and is easily wrinkled when an electrodes-group is produced.Because the separator has a high density, it is difficult to diffuse apoured electrolyte into the separator, and thus, the handlingcharacteristics are poor when an electrodes-group is produced.

The inventors of the present invention made an intensive investigationof an alkaline battery separator which can remedy the abovedisadvantages of prior art, and can be manufactured in a good yield witha good workability. As a result, the inventors found that when aseparator mainly comprising hydrophilicity-imparted, relatively thickand stiff polyolefin fibers having a fiber diameter of 8 μm or morewherein a part of the hydrophilicity-imparted polyolefin fibers is ofhigh-strength fibers having a tensile strength of 5 g/d or more is used,a short circuit caused by a puncturing of the separator by a flashoccurring at the electrode is avoided, and the separator is not torn byan edge of an electrode. Further, a battery can be stably produced in agood yield with a good workability, while the separator has the desiredstiffness and is not wrinkled. Further, the present inventors also foundthat the separator mainly comprising hydrophilicity-imparted, relativelythick and stiff polyolefin fibers having a fiber diameter of 8 μm ormore can provide spaces sufficient for holding an electrolyte, and theproperty required to diffuse a poured electrolyte is excellent.

Accordingly, the object of the present invention is to provide analkaline battery separator which can be produced in a good yield with agood workability, and a process for producing the same.

DISCLOSURE OF INVENTION

The present invention relates to an alkaline battery separatorcharacterized in that the separator comprises a fiber sheet mainlycomprising hydrophilicity-imparted polyolefin fibers having a fiberdiameter of 8 μm or more is contained, and a part of thehydrophilicity-imparted polyolefin fibers is composed of high-strengthfibers having a tensile strength of 5 g/d or more. The alkaline batteryseparator of the present invention will be sometimes referred to assimply a “separator” hereinafter.

Further, the present invention also relates to a process for producingan alkaline battery separator comprising the steps of:

-   -   forming a fiber sheet from polyolefin fibers which have a fiber        diameter of 8 μm or more and contain polyolefin high-strength        fibers having a fiber diameter of 8 μm or more and a tensile        strength of 5 g/d or more; and then imparting a hydrophilic        property to the resulting fiber sheet.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be explained in detail hereinafter.

A fiber sheet forming the alkaline battery separator of the presentinvention mainly comprises a polyolefin fibers which have a fiberdiameter of 8 μm or more and are treated to impart a hydrophilicproperty (i.e., hydrophilicity-imparted polyolefin fibers). Because thefiber sheet mainly comprises hydrophilicity-imparted polyolefin fibers,it has a high alkali resistance. Further, the fiber sheet has anexcellent electrolyte-holding capacity, because it is treated to imparta hydrophilic property. The expression “mainly comprise” as used hereinmeans that the subject fibers, i.e., hydrophilicity-imparted polyolefinfibers having a fiber diameter of 8 μm or more, account for more than 75mass %, preferably 80 mass % or more, more preferably 85 mass % or more,more preferably 90 mass % or more, more preferably 95 mass % or more,most preferably 100 mass %, with respect to constituent fibers of thefiber sheet.

The polyolefin fibers before the treatment for imparting a hydrophilicproperty may contain a resin component, for example, polymers frommonomers, such as propylene, ethylene, butene, or methyl pentene,copolymers of two or more kinds of the monomers as above, or copolymersof the above monomers and vinyl alcohol, acrylic acid or methacrylicacid, such as ethylene vinyl alcohol copolymers, ethylene-acrylic acidcopolymers, or ethylene-methacrylic acid copolymers. The polyolefinfiber may be composed of one kind of a resin component as above, or maybe a composite fiber comprising two or more resin components as above.As the composite fiber, there may be mentioned, for example, a compositefiber whose sectional shape is a sheath-core type, a side-by-side type,an eccentric type, an islands-in-sea type, an orange type or a multiplebimetal type.

A fiber diameter of each fiber of the hydrophilicity-imparted polyolefinfibers contained as the main component in the fiber sheet forming thealkaline battery separator of the present invention is not limited, aslong as it is 8 μm or more. However, it is preferably 9 μm or more, morepreferably 10 μm or more, more preferably 10.5 μm or more, morepreferably 11 μm or more, more preferably 12 μm or more, more preferably12.5 μm or more, most preferably 13 μm or more. For a fiber having anon-circular sectional shape, a diameter of a circle having an area thesame as that of the non-circular sectional shape is regarded as adiameter in the present specification.

Because the fiber sheet forming the alkaline battery separator of thepresent invention mainly comprise relatively thickhydrophilicity-imparted polyolefin fibers as above, i.e.,hydrophilicity-imparted polyolefin fibers having a required stiffness,the fiber sheet has a good stiffness, a good workability, and anexcellent compression elasticity. Therefore, a short circuit causedbetween electrodes by a puncturing of the separator by a flash occurringon the electrode is avoided, a separator is not torn by an edge of anelectrode, and a battery can be stably produced in a good yield.Further, the fiber sheet forming the alkaline battery separator of thepresent invention mainly comprises relatively thick, i.e., stiff,hydrophilicity-imparted polyolefin fibers having a fiber diameter of 8μm or more, and can maintain spaces sufficient for holding anelectrolyte. Therefore, the fiber sheet has an excellent propertyrequired to diffuse a poured electrolyte, that is, the pouredelectrolyte can be rapidly diffused all over the battery. The upperlimit of the fiber diameter is not limited, but preferably isapproximately 30 μm. When the fiber diameter is 30 μm or less, theelectrolyte-holding capacity is not lowered.

Embodiments of the fiber sheet forming the alkaline battery separator ofthe present invention are not limited, as long as the fiber sheet mainlycomprises hydrophilicity-imparted polyolefin fibers. The fiber sheet maybe, for example, a woven fabric, a knitted fabric, or anon-woven-fabric, or a composite fabric thereof. Of these fiber sheets,a fiber sheet containing the non-woven fabric is preferable, becausefibers may be placed three-dimensionally, and it has an excellentelectrolyte-holding capacity.

The fiber sheet which is a constituent component of the alkaline batteryseparator of the present invention contains, as a part ofhydrophilicity-imparted polyolefin fibers, hydrophilicity-impartedpolyolefin high-strength fibers having a tensile strength of 5 g/d ormore, preferably 7 g/d or more, more preferably 9 g/d or more, mostpreferably 12 g/d or more; 50 g/d or less is appropriate. Thehydrophilicity-imparted polyolefin high-strength fiber will be sometimesreferred to as “high-strength fiber” hereinafter. The fiber sheet maycontain only one kind of the high-strength fibers, or two or more kindsof the high-strength fibers. The fiber sheet contains the high-strengthfibers, and thus a separator will not be punctured by a flash occurringon the electrode or torn by an edge of an electrode when anelectrodes-group is produced. The electrodes-group can be produced in agood yield. The fiber sheet containing the high-strength fibers has adesired stiffness, and thus is not wrinkled and has a good workability.The term “tensile strength” of fibers as used herein means a valuemeasured in accordance with JIS L1015 (Japanese Industrial Standard), atesting method for man-made staple fibers.

The high-strength fibers may be composed of resins such as polypropyleneor ultra-high-molecular-weight polyethylene. The term“ultra-high-molecular-weight” as used herein with regard to“ultra-high-molecular-weight polyethylene” means that a weight-averagemolecular weight thereof is 1,000,000 or more. The weight-averagemolecular weight of the ultra-high-molecular-weight polyethylene isgenerally 1,000,000 to 5,000,000. These high-strength fibers may beobtained commercially and are readily available.

The high-strength fibers may be composed of such a resin componentalone, i.e., only one kind of a resin component, or may be obtained bymixing or combining two or more resin components. A sectional shape ofthe latter composite high-strength fibers is, for example, asheath-core, eccentric, laminate, islands-in sea, orange, ormultibimetal type. The composite high-strength fibers from two or moreresin components as above is preferable, because such fibers can befused with a resin component forming the surface of fibers and the 5%modulus strength can be improved. Further, high-strength fibers whichcan be fused with a resin component forming the whole surface of fibers,for example, sheath-core, eccentric, or islands-in sea typehigh-strength fibers are preferable, particularly sheath-core typehigh-strength fibers are more preferable. As the preferable sheath-coretype high-strength fibers, sheath-core type high-strength fiberscontaining a polypropylene resin or ultra-high-molecular-weightpolyethylene as a core component and a polyethylene resin having a lowermelting point than that of the core component resin as a sheathcomponent is most preferable, because the melting points between thecore and sheath components is high, and the tensile strength of thehigh-strength fibers can be maintained. As described above, thehigh-strength fibers may be fusible high-strength fibers or non-fusiblehigh-strength fibers.

A fiber diameter of each fiber of the high-strength fibers is notlimited, as long as it is 8 μm or more. Approximately 8 to 30 μm ispreferable, approximately 9 to 22 μm is more preferable, approximately10 to 22 μm is more preferable, approximately 10.5 to 22 μm is morepreferable, approximately 11 to 22 μm is more preferable, approximately12 to 22 μm is more preferable, approximately 12.5 to 22 μm is morepreferable, approximately 13 to 22 μm is more preferable, andapproximately 13 to 18 μm is most preferable. When the fiber diameter ofthe high-strength fibers is approximately 8 to 30 μm, a short circuitcaused during the production of the electrodes-group is effectivelyprevented, an excellent resistance to tearing is obtained, and anelectrolyte-holding capacity and a property required to diffuse a pouredelectrolyte are not lowered.

A fiber length of the high-strength fibers contained in the fiber sheet,preferably in the fiber sheet containing the non-woven fabric, ispreferably 1 to 60 mm, more preferably 3 to 25 mm, most preferably 5 to20 mm. When the fiber length of high-strength fibers is 1 to 60 mm, afiber sheet having the maximum pore diameter of 50 μm or less,preferably a fiber sheet wherein pores having a pore diameter of 30 μmor less account for 95% or more with respect to the whole of pores, maybe easily prepared. When the maximum pore diameter in the fiber sheet is50 μm or less, preferably when pores having a pore diameter of 30 μm orless therein account for 95% or more with respect to the whole of pores,powdery active materials which may fall from the electrodes cannoteasily penetrate into inner spaces of the separator when the separatoris heavily pressed against the electrode, and thus a short circuitrarely occurs.

The fiber sheet preferably contains 10 mass % or more, more preferably20 mass % or more, most preferably 30 mass % or more of thehigh-strength fibers. When the fiber sheet contains 10 mass % or more ofhigh-strength fibers, a short circuit caused during the production ofthe electrodes-group is effectively prevented, an excellent resistanceto tearing is obtained, and required stiffness also obtained.

In addition to the above high-strength fibers, the fiber sheet formingthe alkaline battery separator of the present invention may containhydrophilicity-imparted polyolefin fusible fibers which can be fusedwith each other and have a tensile strength of less than 5 g/d, i.e.,hydrophilicity-imparted polyolefin fusible low-strength fibers(hereinafter referred to as “fusible fibers”), as one of other fibers.The fiber sheet may contain only one kind of the fusible fibers, or twoor more kinds of the fusible fibers. When the fiber sheet containsfusible fibers, a tensile strength or stiffness of the fiber sheet isimproved, and thus the workability is also improved. Therefore, thebattery can be produced in a good yield, without breaking upon theproduction of the electrodes-group. The fusible fiber preferablycontains, as a component forming at least a fiber surface, a fusiblecomponent having a melting point lower than that of the high-strengthfiber, preferably lower by 10° C. or more, more preferably lower by 15°C. or more. If the high-strength fiber contains two or more resincomponents, the melting point thereof means the highest melting point.In this case, the tensile strength of the high-strength fibers is notlowered.

When the high-strength fibers comprise polypropylene orultra-high-molecular-weight polyethylene, there may be mentioned, as thefusible component, for example, polyethylene resins, for example,low-density polyethylene, linear low-density polyethylene, high-densitypolyethylene, or polyethylene copolymers, such as ethylene-acrylic acidcopolymer or ethylene-methacrylic acid copolymer.

The fusible fibers may be composed of the above resin component alone,or preferably may be composed of two or more resin components wherein afusible component is exposed on the surface of fibers, because a fibrousshape can be maintained by resin components other than the fusiblecomponent. When the fusible fiber is composed of two or more resincomponents, a sectional shape of the fusible fiber may be, for example,a sheath-core, eccentric, side-by-side, islands-in sea, multibimetal, ororange type. The fusible fibers may be easily spun in accordance with,for example, a composite spinning method or a mixing spinning method, ora combination thereof, or are commercially and easily available. Whenthe fusible fiber contains two or more resin components, resincomponents other than the fusible component are not limited, as long asthe resin components other than the fusible component have a meltingpoint higher than that of the fusible component, preferably higher by10° C. or more, more preferably higher by 20° C. or more.

A fiber diameter of each fiber of the fusible fibers is not limited, aslong as it is 8 μm or more. Approximately 8 to 30 μm is preferable,approximately 9 to 22 μm is more preferable, approximately 10 to 22 μmis more preferable, approximately 10.5 to 22 μm is more preferable,approximately 11 to 22 μm is more preferable, approximately 12 to 22 μmis more preferable, approximately 12 to 18 μm is more preferable,approximately 12.5 to 18 μm is more preferable, and approximately 13 to18 μm is most preferable. When the fiber diameter of fusible fibers isapproximately 8 to 30 μm, the electrolyte-holding capacity is notlowered.

When the fiber sheet forming the separator contains the high-strengthfibers (including the fusible high-strength fibers and/or thenon-fusible high-strength fibers) and the fusible fibers, the fiberdiameter of the high-strength fibers is preferably equal to or longerthan that of the fusible fibers, more preferably 1 to 2 fold, mostpreferably 1 to 1.6 fold, as this enables appropriate spaces for asealed-type alkaline battery separator to be formed.

A fiber length of the fusible fibers contained in the fiber sheet(preferably the fiber sheet containing the non-woven fabric) ispreferably 1 to 60 mm, more preferably 3 to 25 mm, most preferably 5 to20 mm, because it is thus easy to prepare a fiber sheet wherein themaximum pore diameter of the fiber sheet is 50 μm or less, or preferablypores having a pore diameter of 30 μm or less in the fiber sheet accountfor 95% or more with respect to the whole of pores.

The fiber sheet preferably contains 20 mass % or more, more preferably30 mass % or more, of fusible fibers. When the fiber sheet contains 20mass % or more of fusible fibers, the tensile strength or stiffness ofthe separator is improved. The fiber sheet preferably contains 20 mass %or more, more preferably 30 mass % or more, of fibers having fusibility(including the fusible fibers and/or fusible high-strength fibers).

When fibers forming the fiber sheet contain the non-fusiblehigh-strength fibers and the fusible fibers, the mass ratio of thenon-fusible high-strength fibers and the fusible fibers is preferably 10to 50:90 to 50, more preferably 20 to 40:80 to 60, most preferably 30 to40:70 to 60.

When the fiber sheet contains the fusible high-strength fibers, thefusible high-strength fibers preferably account for 10 mass % or more ofthe fiber sheet.

The fiber sheet forming the alkaline battery separator of the presentinvention may further contain fibers other than the above high-strengthfibers or the above fusible fibers. For example, the fiber sheet mayfurther contain non-fusible polyolefin fibers having a tensile strengthof less than 5 g/d and treated to impart a hydrophilic property, i.e.,hydrophilicity-imparted polyolefin non-fusible low-strength fibers.Hereinafter referred to as “non-fusible low-strength fibers”. The fibersheet may contain a kind of non-fusible low-strength fibers, or two ormore kinds of non-fusible low-strength fibers. A fiber diameter of eachfiber of the non-fusible low-strength fibers is not limited, as long asit is 8 μm or more. Approximately 8 to 30 μm is preferable,approximately 9 to 22 μm is more preferable, approximately 10 to 22 μmis more preferable, approximately 10.5 to 22 μm is more preferable,approximately 11 to 22 μm is more preferable, approximately 12 to 22 μmis more preferable, approximately 12 to 18 μm is more preferable,approximately 12.5 to 18 μm is more preferable, and approximately 13 to18 μm is most preferable. When the fiber diameter is approximately 8 to30 μm, the electrolyte-holding capacity is not lowered.

In the fiber sheet forming the alkaline battery separator of the presentinvention, it is preferable that polyethylene fibers wherein the surfacethereof consists essentially of polyethylene resins account for 60 mass% or more, more preferably 65 mass % or more, most preferably 70 mass %or more, of the hydrophilicity-imparted polyolefin fibers (i.e., thehigh-strength fibers, the fusible fibers, and /or the non-fusiblelow-strength fibers) forming the fiber sheet. This is because thetreatment for imparting a higher degree of the hydrophilicity can bemore easily carried out, and thus an excellent electrolyte-holdingcapacity is obtained, and therefore, a battery having a long lifetimecan be produced. As the polyethylene resin, there may be mentioned, forexample, low-density polyethylene, linear low-density polyethylene,high-density polyethylene, ultra-high-molecular-weight polyethylene, orpolyethylene copolymers, such as ethylene-acrylic acid copolymer orethylene-methacrylic acid copolymer.

The maximum pore diameter in the fiber sheet (preferably the fiber sheetcontaining the non-woven fabric) forming the alkaline battery separatorof the present invention is preferably 50 μm or less, more preferably 45μm or less, most preferably 40 μm or less. When the maximum porediameter is 50 μm or less, powdery active materials which may fall fromthe electrodes cannot easily penetrate into inner spaces of theseparator, when the separator is heavily pressed against the electrode,and thus a short circuit rarely occurs. The term “maximum pore diameter”as used herein means a value measured in accordance with a bubble pointmethod, using a porometer (Coulter Electronics Ltd.).

The maximum pore diameter of 50 μm or less in the fiber sheet can beobtained by adjusting various factors, for example, a fineness of eachfiber (including the high-strength fibers) forming the fiber sheet, afiber length of each fiber (including the high-strength fibers) formingthe fiber sheet, a degree of fusions when the fusible high-strengthfibers and/or fusible fibers are fused, a sectional shape of each fiberforming the fiber sheet, a mass per unit area of the fiber sheet, athickness of the fiber sheet, and so on.

A 5% modulus strength with respect to at least a direction of the fibersheet (preferably a fiber sheet containing a non-woven fabric) formingthe alkaline battery separator of the present invention is preferably a60 N/5 cm width or more, more preferably a 70 N/5 cm width or more, mostpreferably a 80 N/5 cm width or more. When the 5% modulus strength is a60 N/5 cm width or more, a distortion of the separator may be avoided,and a change of each pore size in the separator may be small, even ifthe separator is strongly pressed against the electrodes. Further,powdery active materials which may fall from the electrodes cannoteasily penetrate into inner spaces of the separator, and thus a shortcircuit rarely occurs. The term “5% modulus strength” as used hereinmeans a force required to extend a separator by 5 mm, by pulling a fibersheet (for example, a non-woven fabric) cut to a 5 cm width which hasbeen set between chucks (distance between the chucks=100 mm) of atensile tester (TENSILON UTM-III-100; manufactured by ORIENTEC, Co.), ata pulling rate of 300 mm/min.

The 5% modulus strength of the fiber sheet can be made a 60 N/5 cm widthor more, by adjusting various factors, for example, a fineness of eachfiber (including the high-strength fibers) forming the fiber sheet, afiber length of each fiber (including the high-strength fibers) formingthe fiber sheet, an orientation of fibers (including the high-strengthfibers) forming the fiber sheet, a degree of fusing when the fusiblehigh-strength fibers and/or fusible fibers are fused, a degree ofentanglement of fibers forming the fiber sheet, and so on.

In the fiber sheet, preferably a fiber sheet containing a non-wovenfabric, forming the alkaline battery separator of the present invention,pores having a pore diameter of 30 μm or less preferably account for 95%or more of the whole pores in the fiber sheet, more preferably 100%,i.e., all of the pores have a pore diameter of 30 μm or less. When poreshaving a pore diameter of 30 μm or less account for 95% or more of thewhole pores in the fiber sheet, powdery active materials falling fromthe electrodes cannot easily penetrate into inner spaces of theseparator, and thus a short circuit is rarely occurs even if theseparator is strongly pressed against the electrodes. The “ratio ofpores having a pore diameter of 30 μm or less of the whole pores” in thepresent specification may be calculated by measuring a pressure of gas(nitrogen gas) in accordance with a bubble point method, calculating apore diameter in accordance with a Washburn equation, and calculating adistribution of the pore diameter from a flow of the gas.

The fiber sheet wherein the pores having a pore diameter of 30 μm orless account for 95% or more of the whole pores can be obtained byadjusting various factors, for example, a fineness of each fiber(including the high-strength fibers) forming the fiber sheet, a fiberlength of each fiber (including the high-strength fibers) forming thefiber sheet, a degree of fusing when the fusible high-strength fibersand/or fusible fibers are fused, a sectional shape of each fiber formingthe fiber sheet, a mass per unit area of the fiber sheet, a thickness ofthe fiber sheet, and so on.

The fiber sheet (preferably a fiber sheet containing a non-woven fabric)forming the alkaline battery separator of the present invention has anair permeability of, preferably 4 cm/sec or more, more preferably 6cm/sec or more, most preferably 8 cm/sec or more. The upper limit of theair permeability of the fiber sheet is not limited, but is preferably 50cm/sec or less. When the air permeability is 4 cm/sec or more, gasgenerated in one electrode can easily migrate to the other electrode,and thus the separator having an air permeability of 4 cm/sec or moremay be preferably used in a sealed-type alkaline battery. The term “airpermeability” as used herein means a value measured in accordance withthe method defined in JIS L 1096 (1999) [8.27.1 A method (Frazirmethod)].

The fiber sheet having an air permeability of 4 cm/sec or more can beobtained by adjusting various factors, for example, a fineness of eachfiber (including the high-strength fibers) forming the fiber sheet, afiber length of each fiber (including the high-strength fibers) formingthe fiber sheet, a degree of fusing when the fusible high-strengthfibers and/or fusible fibers are fused, a degree of entanglement offibers forming the fiber sheet, a mass per unit area of the fiber sheet,a thickness of the fiber sheet, and so on.

An electrical resistance of the fiber sheet (preferably a fiber sheetcontaining a non-woven fabric) forming the alkaline battery separator ofthe present invention is preferably 5 mΩ·100 cm²/sheet or less, morepreferably 3 mΩ·100 cm²/sheet or less, most preferably 2 mΩ·100cm²/sheet or less. The lower limit of the electrical resistance of thefiber sheet is not limited, but is preferably 10⁻³ mΩ·100 cm²/sheet ormore. When the electrical resistance is 5 mΩ·100 cm²/sheet or less, aninner resistance of the alkaline battery is reduced, and therefore, abattery having an excellent battery capacity and charge-dischargecharacteristics may be obtained.

The fiber sheet having an electrical resistance of 5 mΩ·100 cm²/sheet orless can be obtained, for example, by increasing a void volume rate toenlarge spaces capable of holding an electrolyte; locating hydrophilicgroups, such as a sulfonic acid, hydroxyl, or carboxyl group, on thesurface of fibers forming the fiber sheet; thinning a thickness of theseparator; or the like.

The term “electrical resistance” as used herein means a value obtainedin accordance with the procedure concretely disclosed in Examples asbelow, using a test equipment disclosed in JIS C2313 (Separator for alead secondary battery), 7.2.4 Electrical Resistance, 3. Test Equipment.

A process for producing the fiber sheet forming the alkaline batteryseparator of the present invention is not limited, but the fiber sheetmay be prepared, for example, (1) by forming a fiber sheet fromhydrophilicity-imparted polyplef in fibers obtained by imparting ahydrophilic property to polyolefin fibers; (2) carrying out fiber sheetforming steps to any intermediate step where a step to impart ahydrophilic property is carried out, and then performing the remainingsteps to obtain a fiber sheet; or (3) forming a fiber sheet frompolyolefin fibers to which a hydrophilic property had not been imparted,and imparting a hydrophilic property to the resulting fiber sheet.Because fibers are not damaged during the preparation of a fiber sheet,it is preferable to prepare the separator by the method wherein thetreatment to impart a hydrophilic property is carried out after formingthe fiber sheet. An embodiment wherein a fiber sheet is composed of anon-woven fabric, and a hydrophilic property is imparted after formingthe fiber sheet will be described hereinafter. However, a treatment toimpart a hydrophilic property may be carried out as in the followingembodiment, for the fibers before forming the fiber sheet, or at anyintermediate step during the formation of the fiber sheet.

A non-woven fabric preferable as a fiber sheet may be prepared from, forexample, polyolefin fibers having a fiber diameter of 8 μm or more andcontaining polyolefin high-strength fibers having a fiber diameter 8 μmor more and a tensile strength of 5 g/d or more, in accordance with amethod wherein a fiber web is formed by a dry-laid method, such as acarding method, an air-laid method, a spun-bonding method, or amelt-blown method, or a wet-laid method, and then entangled by a fluidjet, such as a water jet; a method wherein fusible fibers and/or fusiblehigh-strength fibers are mixed in a fiber web and fused; or a methodwherein a web is bonded with a binder; or a combination thereof.

When a non-woven fabric having a maximum pore diameter of 50 μm or lessis prepared, preferably a non-woven fabric wherein pores having a porediameter of 30 μm or less account for 95% or more with respect to thewhole pores is prepared, the wet-laid method is preferable for formingthe fiber web. As the wet-laid method, there may be mentionedconventional methods, such as a flat long-wire type, a flat short-wiretype, an inclined long-wire type, an inclined short-wire type, acylinder type, a long-wire cylinder combination type, or ashort-wire-cylinder combination type. When the fiber web is formed bythe wet-laid method, the resulting non-woven fabric has a tendency toshow a lower 5% modulus strength. Therefore, it is preferable thatfibers are orientated in a nearly unidirectional manner by adjusting amoving rate of a net for laying fibers and an amount of a slurry flow soas to obtain a separator showing a 5% modulus strength of a 60 N/5 cmwidth or more at least a direction thereof.

A method for entangling fibers may be, for example, a method forejecting a fluid jet, particularly a water jet, to a fiber web. Themethod for ejecting a fluid jet is preferable, because a degree ofentanglement is high, a 5% modulus strength of the separator is high,and a whole fiber web can be uniformly entangled.

Specifically, a fluid jet under a pressure of 1 to 30 MPa may be ejectedonto a fiber web from a nozzle plate containing one or more lines ofnozzles having a diameter of 0.05 to 0.3 mm and a pitch of 0.2 to 3 mm.The fluid jet may be applied to one side or both sides of the fiber web,once or more times. If a supporter, such as a net, to carry a fiber webthereon when treated with the fluid jet contains thick supportingportions (non-opening portions), the resulting separator contains poreshaving a large diameter, and a short circuit is liable to occur.Therefore, it is preferable to use a supporter which contains supportingportions having a thickness of 0.25 mm or less.

Of the above-mentioned known methods for preparing a non-woven fabric, apreferable method comprises mixing fusible fibers and/or fusiblehigh-strength fibers in a fiber web, and fusing the fusible fibersand/or fusible high-strength fibers in such a manner that a tensilestrength and stiffness of the resulting non-woven fabric are improved.Namely, a tensile strength and stiffness of the resulting non-wovenfabric are improved by the above method. The fiber web used may becomposed of a single layer or multiple layers of same or different fiberwebs. For example, a non-woven fabric having a tensile strength anduniformity can be prepared from a laminated fiber web comprising a fiberweb prepared by a dry-laid method and a fiber web prepared by a wet-laidmethod.

The fusing treatment can be carried out under pressure or withoutpressure, or first without pressure to fuse the fusible component andthen under pressure. When the fusing treatment is carried out withoutpressure, it is preferably carried out within the range of from asoftening temperature of the fusible component in fusible fibers and/orfusible high-strength fibers to a temperature which is 20° C. higherthan a melting point of the fusible component. When the fusing treatmentis carried out under pressure, it is preferably carried out within therange of from a softening temperature to a melting point of the fusiblecomponent in fusible fibers and/or fusible high-strength fibers. Thefusing treatment may be carried out by a heating calender, a hot-airthrough-type heater, or a cylinder contact heater. A linear pressureapplied when a heat is applied under pressure, or when a heat is appliedfirst and thereafter a pressure is applied, is preferably approximately5 to 30 N/cm. The term “melting point” as used herein means atemperature of a maximum value in a melting-endothermic curve obtainedby raising the temperature from room temperature at a rate of 10°C./min, using a differential scanning calorimeter. Further, the term“softening point” as used herein means a temperature of a starting pointin a melting-endothermic curve obtained by raising the temperature fromroom temperature at a rate of 10° C./min, using a differential scanningcalorimeter.

The entangling treatment or the fusing treatment may be carried out inan alternative manner, or the entangling treatment and the fusingtreatment may be used in combination thereof, to enhance the 5% modulusstrength of the separator. Further, the number of the entangling and/orfusing treatments to be carried out, and the sequential order of theentangling and/or fusing treatments are not limited. It is preferable tocarry out first the entangling treatment and then the fusing treatment,because a fused structure formed by the fusing treatment cannot be thendestroyed by the entangling treatment. In this case, a highly entangledstructure is fused, and thus a 5% modulus strength is further improved.

The treatment used for imparting a hydrophilic property may be, forexample, a sulfonating treatment, a treatment with fluorine gas, a graftpolymerization treatment with vinyl monomers, a treatment with asurface-active agent, a discharging treatment, or a treatment to adherehydrophilic resins.

The sulfonating treatment contains, for example, but is not limited to,a treatment with fuming sulfuric acid, sulfuric acid, sulfur trioxide,chlorosulfuric acid, or sulfuryl chloride. Of these treatments, thesulfonating treatment with fuming sulfuric acid is preferable, becauseof a high reactivity and an easier sulfonation is thus obtained.Sulfonic acid groups may be introduced into polyolefin fibers by thesulfonating treatment.

The treatment with fluorine gas includes, for example, but is notlimited to, the treatment with a gas mixture of fluorine gas dilutedwith an inactive gas (such as nitrogen, argon, or helium gas) and atleast one gas selected from a the group consisting of oxygen, carbondioxide and sulfur dioxide gases. Fluorine, oxygen, or sulfur atoms, orfunctional groups containing these atoms (such as a sulfonic acid group)may be introduced into polyolefin fibers by the treatment with fluorinegas.

Examples of the vinyl monomer which may be used in the graftpolymerization treatment are acrylic acid, methacrylic acid, acrylate,methacrylate, vinyl pyridine, vinyl pyrrolidone, or styrene. Acrylicacid has a good affinity with an electrolyte and may be preferably used.

The vinyl monomers can be polymerized, for example, by dipping thenon-woven fabric in a solution containing the vinyl monomers and aninitiator, and heating; by coating the non-woven fabric with vinylmonomers and applying radiation; by applying radiation to the non-wovenfabric and then bringing the non-woven fabric into contact with thevinyl monomers, by impregnating the non-woven fabric with a solutioncontaining vinyl monomers and a sensitizing agent, and applyingultraviolet rays. The graft polymerization can be effectively carriedout by modifying the surface of fibers with ultraviolet radiation, acorona discharge or a plasma discharge before bringing the non-wovenfabric into contact with the vinyl monomer solution, to enhance theaffinity thereof with the vinyl monomer solution. When the vinylmonomers are polymerized by applying radiation or ultraviolet radiation,it is preferable to apply radiation or ultraviolet radiation first inthe presence of oxygen, but subsequently under the condition that thenon-woven fabric is surrounded by an air-nonpermeable film or thecondition that all the [1 surfaces of the non-woven fabric are coveredover with an air-nonpermeable film, i.e., the condition that oxygen isnot excluded. This improves a resistance to oxidation.

The treatment with a surface-active agent may be carried out byapplying, for example, spraying or coating, a solution of an anionicsurface-active agent (such as an alkali metal-salt of a higher fattyacid, alkyl sulfonate, or a salt of sulfosuccinate) or a nonionicsurface-active agent (such as polyoxyethylene alkyl ether, orpolyoxyethylene alkylphenol ether) to the non-woven fabric, or dippingthe non-woven fabric in the solution.

As the discharging treatment, there may be mentioned, for example,treatments with a corona discharge, plasma, glow discharge, surfacedischarge, or electron rays. Of the discharging treatments, the plasmatreatment comprising the steps of placing the non-woven fabric between apair of electrodes carrying a dielectric layer respectively in air underan atmospheric pressure, so that the non-woven fabric is brought intocontact with both dielectric layers, and then applying an alternatingcurrent voltage between the electrodes to thereby induce an electricdischarge in internal voids contained in the non-woven fabric, can bepreferably conducted. This is because not only the outer surfaces butalso the insides of the non-woven fabric can be thus modified; andtherefore, a battery having an excellent electrolyte-holding capacity inthe separator, and an excellent inner pressure characteristic in anexcellent oxygen-absorbability upon overcharging can be produced.

The treatment to adhere hydrophilic resins can be carried out byadhering hydrophilic resins, such as carboxymethyl cellulose, polyvinylalcohol, polyvinyl alcohol which is cross-linkable, or polyacrylic acid,to the non-woven fabric. The hydrophilic resins may be adhered to thenon-woven fabric by spraying or coating the non-woven fabric with asolution prepared by dissolving or dispersing the hydrophilic resins inan appropriate solvent, or dipping the non-woven fabric in the solution,and then drying. The amount of the hydrophilic resins adhered ispreferably 0.1 to 5 mass % with respect to an amount of the wholeseparator after adherence. In this case, the air permeability is notaffected.

The polyvinyl alcohol which is cross-linkable, for example, polyvinylalcohol substituted by a photosensitive group at a part of hydroxygroups, particularly polyvinyl alcohol having styryl pyridinium, styrylquinolinium, or styryl benzthiazolinium groups as the photosensitivegroup. The cross-linkable polyvinyl alcohol can be cross-linked as inthe case of other hydrophilic resins, i.e., by adhering to the non-wovenfabric and irradiating with light. The polyvinyl alcohol substituted bya photosensitive group at a part of hydroxy groups has an excellentresistance to alkalis and contains many hydroxyl groups, and thus canform chelates with ions before the ions are deposited on electrodes inthe form of branches, during charging and/or discharging, to therebyeffectively prevent a short circuit between the electrodes.

The mass per unit area of the alkaline battery separator of the presentinvention is preferably 30 to 100 g/m², more preferably 40 to 80 g/m².If the mass per unit area is less than 30 g/m², a sufficient tensilestrength may not be obtained. If the mass per unit area is more than 100g/m², the separator becomes too thick to obtain a battery with a highcapacity.

The alkaline battery separator of the present invention can be used as aseparator of an alkaline primary battery, such as an alkaline-manganesebattery, a mercury battery, a silver oxide battery, an air battery, orthe like, or an alkaline secondary battery, such as a nickel cadmiumbattery, silver-zinc battery, silver-cadmium battery, nickel-zincbattery, nickel-hydrogen battery or the like, particularly a separatorof a nickel-cadmium battery or nickel-hydrogen battery.

EXAMPLES

The present invention now will be further illustrated by, but is by nomeans limited to, the following Examples.

Example 1

A fiber web was formed from a slurry prepared by mixing and dispersing30 mass % of polypropylene high-strength fibers [tensile strength=9 g/d,fineness=2 denier (fiber diameter=17.7 μm), fiber length=10 mm, meltingpoint=160° C.] and 70 mass % of sheath-core type fusible fibers[fineness=1.1 denier (fiber diameter=13.1 μm), fiber length=10 mm]containing a polypropylene core component (melting point=157° C.) and alow-density polyethylene sheath component (melting point=115° C.) by awet-laid method.

The fiber web was heated at 120° C. without pressure for 10 seconds, andpassed through calendering rolls under a linear pressure of 9.8 N/cm, tofuse only the low-density polyethylene component in the sheath-core typefusible fibers and prepare a non-woven fabric.

Then, the non-woven fabric was placed in a container filled with a gasmixture containing fluorine (3 vol %), oxygen (5 vol %), sulfur dioxide(5 vol %), and nitrogen (87 vol %) gases, and brought into contact withthe gas mixture for 120 seconds to impart a hydrophilic property. Analkaline battery separator (mass per unit area=60 g/m², thickness=0.15mm) of the present invention was obtained.

Example 2

The non-woven fabric was prepared as in Example 1, and dipped in asolution of fuming sulfuric acid (15% SO₃ solution) at 39° C. for 10minutes to impart a hydrophilic property. An alkaline battery separator(mass per unit area=60 g/m², thickness=0.15 mm) of the present inventionwas obtained.

Example 3

The non-woven fabric was prepared as in Example 1, and then dipped inthe graft-polymerizing liquid as described below, and irradiated in airfor 15 seconds with ultraviolet light having a central wavelength of 365nm at a light intensity of 180 mW/cm², by metal halide mercury vaporlamps. A lamp was located for an obverse side, and another lamp waslocated for a reverse side. Thereafter, the non-woven fabric wassandwiched between two gas-nonpermeable films so that air remaining inthe inner spaces and the spaces around the outer surfaces of thenon-woven fabric did not escape. Then, the non-woven fabric wasirradiated for 15 seconds with ultraviolet light having a centralwavelength of 365 nm at a light intensity of 180 mW/cm², by metal halidemercury vapor lamps. A lamp was located for an obverse side, and anotherlamp was located for a reverse side. When the irradiation withultraviolet light was carried out under the condition that the non-wovenfabric was sandwiched between the gas-nonpermeable films, thetemperature of the non-woven fabric was 110° C., and thegraft-polymerizing liquid was able to evaporate and diffuse from theedges of the two gas-nonpermeable films. Then the non-woven fabric wasthoroughly rinsed with water and dried, to obtain an alkaline batteryseparator (mass per unit area=65 g/m², thickness=0.15 mm) of the presentinvention. In the alkaline battery separator, acrylic acid monomers werepolymerized at 7.7% with respect to the non-woven fabric.

Composition of the graft-polymerizing liquid (1) acrylic acid monomer:25 mass % (2) benzophenone: 0.3 mass % (3) iron sulfate: 0.4 mass % (4)nonionic surface active agent: 3 mass % (5) water: 61.3 mass % (6)polyethylene glycol 10 mass % (polymerization degree: 400):

Example 4

The procedures of forming a fiber web and a non-woven fabric, andtreating with the fluorine gas disclosed in Example 1 were repeated,except that 40 mass % of the polypropylene high-strength fibers used inExample 1 and 60 mass % of the sheath-core type fusible fibers used inExample 1 were used, to obtain an alkaline battery separator (mass perunit area 60 g/m², thickness=0.15 mm) of the present invention.

Example 5

Thirty mass % of the polypropylene high-strength fibers used in Example1 except that the fiber length thereof is 45 mm, and 70 mass % of thesheath-core type fusible fibers used in Example 1 except that the fiberlength thereof is 38 mm were carded by a carding machine to form a fiberweb. Then, the procedures of forming a non-woven fabric and treatingwith the fluorine gas disclosed in Example 1 were repeated, to obtain analkaline battery separator (mass per unit area=60 g/m², thickness=0.15mm) of the present invention.

Example 6

The procedures of forming a fiber web and a non-woven fabric, andtreating with the fluorine gas disclosed in Example 1 were repeated,except that 5 mass % of the polypropylene high-strength fibers used inExample 1 and 95 mass % of the sheath-core type fusible fibers used inExample 1 were used, to obtain an alkaline battery separator (mass perunit area=60 g/m², thickness=0.15 mm) of the present invention.

Example 7

The procedures of forming a fiber web and a non-woven fabric, andtreating with the fluorine gas disclosed in Example 1 were repeated,except that 60 mass % of the polypropylene high-strength fibers used inExample 1 and 40 mass % of the sheath-core type fusible fibers used inExample 1 were used, to obtained an alkaline battery separator (mass perunit area=60 g/m², thickness=0.15 mm) of the present invention.

Comparative Example 1

A fiber web was formed from 35 mass % of the polypropylene high-strengthfibers used in Example 1, 25 mass % of the sheath-core type fusiblefibers used in Example 1, and 40 mass % of orange-type dividable fibers[fineness≦2 denier, fiber length=10 mm; dividable into 8 polypropylenefine fibers (sectional shape=approximate triangle; fiber diameter=4.4μm, melting point 160° C.) and 8 low-density polyethylene fine fibers(sectional shape=approximate triangle; fiber diameter=4.3 μm, meltingpoint=115° C.)] by a wet-laid method.

The fiber web was mounted on a plain weave net (mesh opening=0.175 mm),and then the fibers were entangled and the dividable fibers were dividedto generate fine fibers with a water jet from a nozzle plate (nozzlediameter=0.15 mm, pitch=0.8 mm, inner pressure=12 MPa). In this case,each side of the fiber web was treated twice with the water jet Thefiber web was dried to form an entangled non-woven fabric. The fusingtreatment of the entangled non-woven fabric was carried out as inExample 1, that is, the low-density polyethylene component in thesheath-core type fusible fibers and the low-density polyethylene finefibers were fused to obtain a fused non-woven fabric. Then, thetreatment with the fluorine gas was carried out as in Example 1 toobtain a separator (mass per unit area=60 g/m², thickness=0.15 mm) forcomparison.

Comparative Example 2

The procedures of forming a fiber web and a fused non-woven fabric, andtreating with the fluorine gas disclosed in Comparative Example 1 wererepeated, except that 60 mass % of the sheath-core type fusible fibersused in Example 1 and 40 mass % of the dividable fibers used inComparative Example 1 were used, to obtain an alkaline battery separator(mass per unit area=60 g/m², thickness=0.15 mm) for comparison.

Evaluation of Properties

(1) Lengthwise Tensile Strength

Each of the separators cut to 50 mm width was set between chucks(distance between the chucks=100 mm) of a tensile tester (TENSILON UTM-III-100; manufactured by ORIENTEC, Co.), and a lengthwise tensilestrength thereof was measured (pulling rate=300 mm/min). The results areshown in Table 1. As shown in Table 1, the alkaline battery separatorsof the present invention had an excellent tensile strength, and thuswere not broken by the tension generated during battery assembly.

(2) Puncturing Force

A laminate having a thickness of about 2 mm was formed from each of theseparators. A stainless steel jig (thickness=0.5 mm; angle of the bladeedge=60°) mounted on a handy-type compression tester (KES-G5;manufactured by KATO TECH Co., Ltd.) was thrust perpendicularly into thelaminate from the top separator at a rate of 0.01 cm/s, and the forcerequired to cut the top separator was measured. The results are shown inTable 1. As apparent, it is difficult to puncture the alkaline batteryseparators of the present invention. Therefore, a battery can beproduced in a good yield without the occurrence of a short circuit.

(3) Ratio of Maintaining Thickness

A thickness of each of the separators at a load of 500 g was measured bya micrometer (diameter of a spindle=6.35 mm). Then, the thickness ofeach of the separators at a load of 1100 g was measured by themicrometer. The thickness at a load of 1100 g is represented by thepercentage with respect to the thickness at a load of 500 g. The resultsare shown in Table 1. As is apparent, the thickness of the alkalinebattery separators of the present invention is not easily changed evenif a pressure is applied. A pressure applied during the assembly of theelectrodes-group will rarely crush the separator, and a short circuitrarely occurs. The shape of the separator can be maintained against anexpansion or contraction of electrodes during a charging or dischargingof a secondary battery, particularly against the expansion during thecharging of the electrodes. It is expected that a battery with a longlifetime be produced, because dry-out rarely occurs and a short circuitrarely occurs.

(4) Lengthwise Bending Resistance

The lengthwise bending resistance of each of the separators was measuredin accordance with JIS L1096 [bending resistance; the A method]. Theresults are shown in Table 1. As shown in Table 1, the alkaline batteryseparators of the present invention have an excellent bendingresistance, and thus show an excellent workability during the assemblyof the electrodes-group.

(5) Lengthwise Tear Strength

The lengthwise tear strength of each of the separators was measured inaccordance with JIS L 1096 (a method for testing general textiles;trapezoidal tearing strength test). The results are shown in Table 1. Asshown in Table 1, the alkaline battery separators of the presentinvention have an excellent tear strength, and thus are not easily tornby the edge of the electrode. Therefore, a battery can be produced in agood yield.

(6) Ratio of Non-conforming Batteries Produced During the BatteryAssembly

A homogenous alloy was produced in an arc smelting furnace, afteraccurately weighing lanthanum (La) with a purity of 99.5% or more,nickel (Ni) with a purity of 99.5% or more, cobalt (Co) with a purity of99.5% or more, manganese (Mn) with a purity of 99.5% or more, and a meshmetal (Mm) containing 98% or more of rare earth elements so that aresulting alloy has an alloy composition ofLa_(0.2)Mm_(0.8)Ni_(3.8)Co_(0.8)Mn_(0.4) as a hydrogen occlusion alloy.The resulting alloy was heated at 1000° C. for 6 hours under vacuum, andthen pulverized into powder of 400 mesh or less. To 100 g of theresulting powder was added 25 g of an aqueous solution of 2% by weightof polyvinyl alcohol, to obtain a slurry paste. An expanded porousnickel (size=260×38 mm; thickness=0.9 mm; porosity=95 to 96%) wasuniformly filled with the resulting paste, dried, pressed at 500 kg/cm²,and then equipped with a nickel lead wire by spot welding to form anegative electrode. On the other hand, an expanded nickel positiveelectrode (size=214×38 mm, thickness=0.68 to 0.7 mm, theoreticalelectrical quantity=3060 to 3100 mAh) was prepared, as a nickel oxideelectrode, in accordance with a conventional method. Thereafter, each ofthe separators was cut into a specimen of 43×560 mm, and placed betweenthe positive electrode and the negative electrode. The whole was thenrolled to obtain a sealed-type nickel-hydrogen secondary battery havinga C-size.

Subsequently, for the resulting sealed-type nickel-hydrogen secondarybatteries, a ratio of non-conforming batteries was determined inaccordance with a criterion that a battery which shows an electricalresistance of 1 kΩ or less, when 240 V is applied between the positiveand negative electrodes, should be rejected. The ratio was calculatedfrom 10,000 assemblies of the sealed-type nickel-hydrogen secondarybatteries in accordance with the above-mentioned method. The results areshown in Table 1. As apparent from Table 1, an alkaline battery can beeffectively produced with a good yield, when the alkaline batteryseparator of the present invention is used.

TABLE 1 A B C D E F Example 1 176 900 120 28 5.5 0.2 Example 2 147 90098 25 3.8 0.3 Example 3 176 900 102 30 5.5 0.2 Example 4 176 975 105 285.5 0.1 Example 5 225 900 102 28 7.5 0.5 Example 6 196 750 95 29 7.0 0.6Example 7 98 1013 105 25 4.0 0.4 Comparative Example 1 147 900 88 15 3.10.7 Comparative Example 2 185 700 88 15 3.3 1.5[In Table 1, “A” denotes a tensile strength (unit=N/5 cm width), “B”denotes a penetrating force (unit=kgf), “C” denotes a ratio ofmaintaining thickness (unit=%), “D” denotes a bending resistance(unit=mg), “E” denotes a tear strength (unit=kg/5 cm width), and “F”denotes a ratio of non-conforming batteries produced during the batteryassembly (unit=%).]

Example 8

A fiber web was formed from a slurry prepared by mixing and dispersing40 mass % of polypropylene high-strength fibers [tensile strength 12g/d, fineness=1.2 denier (fiber diameter=13.7 μm), fiber length=5 mm,melting point=166° C., sectional shape=circular] and 60 mass % ofsheath-core type fusible fibers (fineness=0.7 denier (fiberdiameter=10.3 μm), fiber length=5 mm; rate of the sheath component withrespect to the fiber surface=100%, sectional shape=circular] containinga polypropylene core component (melting point=160° C.) and a low-densitypolyethylene sheath component (melting point=110° C.) by an inclinedlong-wire type method. The fiber web was dried at 135° C., and at thesame time, the sheath component in the sheath-core type fusible fiberswas fused to obtain a fused non-woven fabric (mass per unit area=62g/m², thickness 0.25 mm). When the fiber web was laid, a moving rate ofa net for laying fibers and an amount of a slurry flow was adjusted tounidirectionally orientate fibers. Therefore, the ratio of a lengthwisetensile strength and a crosswise tensile strength of the fused non-wovenfabric became 2:1. The term “tensile strength” as used herein withregard to a separator or fiber sheet, such as a non-woven fabric, meansa force required to break a sample (fused non-woven fabric) cut to a 5cm width, when the sample is set between chucks (distance between thechucks=100 mm) of a tensile tester (TENSILON UTM-III-100; manufacturedby ORIENTEC, Co.) and pulled at a pulling rate of 300 mm/min.

Then, the fused non-woven fabric was passed through rolls at 95° C. tocause a pressure-fusing of the sheath component in the sheath-core typefusible fibers to obtain a pressure-fused non-woven fabric(thickness=0.2 mm). Sulfonic acid groups were introduced into fibersforming the pressure-fused non-woven fabric by dipping the fabric in asolution of fuming sulfuric acid (concentration=15%) at 60° C. for 2minutes to obtain a sulfonated non-woven fabric. The thickness of thesulfonated non-woven fabric was adjusted by calendering at an ordinarytemperature to obtain an alkaline battery separator (mass per unitarea=62 g/m², thickness=0.13 mm) of the present invention.

Example 9

A fiber web was formed from a slurry prepared by mixing and dispersing20 mass % of polypropylene high-strength fibers [tensile strength=12g/d, fineness=2 denier (fiber diameter=17.6 μm), fiber length=10 mm,melting point=166° C., sectional shape=circular] and 80 mass % ofsheath-core type fusible fibers [fineness=1.1 denier (fiberdiameter=13.1 μm), fiber length=10 mm; rate of the sheath component withrespect to the fiber surface≦100%, sectional shape=circular] containinga polypropylene core component (melting point=160° C.) and a low-densitypolyethylene sheath component (melting point=110° C.) by an inclinedlong-wire type method. The fiber web was dried at 135° C., and at thesame time, the sheath component in the sheath-core type fusible fiberswas fused to obtain a fused non-woven fabric (mass per unit area 54g/m², thickness=0.25 mm). When the fiber web was laid, a moving rate ofa net for laying fibers and an amount of a slurry flow was adjusted tounidirectionally orientate fibers. Therefore, the ratio of a lengthwisetensile strength and a crosswise tensile strength of the fused non-wovenfabric became 2:1.

A Then, the fused non-woven fabric was passed through rolls at 95° C. tocause a pressure-fusing of the sheath component in the sheath-core typefusible fibers to obtain a pressure-fused non-woven fabric(thickness=0.2 mm). The pressure-fused non-woven fabric was dipped inthe graft-polymerizing liquid as described below (80 amounts of theliquid with respect to 100 amounts of a mass per unit area of thepressure-fused non-woven fabric), and irradiated in air for 15 secondswith ultraviolet light having a central wavelength of 365 nm at a lightintensity of 180 mW/cm², by metal halide mercury vapor lamps located onboth sides of the pressure-fused non-woven fabric, respectively. Thatis, a lamp was located for an obverse side, and another lamp was locatedfor a reverse side. Thereafter, the pressure-fused non-woven fabricirradiated with ultraviolet light was sandwiched between twogas-nonpermeable films so that air remaining in the inner spaces and thespaces around the outer surfaces of the non-woven fabric did not escape,and irradiated for 15 seconds with ultraviolet light having a centralwavelength of 365 nm at a light intensity of 180 mW/cm², by a metalhalide mercury vapor lamp located on both sides of the pressure-fusednon-woven fabric irradiated with ultraviolet light, respectively, asabove, to obtain a graft-polymerized non-woven fabric (graft ratio=10%).When the irradiation with ultraviolet light was carried out under thecondition that the non-woven fabric was sandwiched between thegas-nonpermeable films, the temperature of the non-woven fabric was 110°C., and the graft-polymerizing liquid was able to evaporate and diffusefrom the edges of two gas-nonpermeable films. The thickness of thegraft-polymerized non-woven fabric was adjusted by calendering at anordinary temperature to obtain an alkaline battery separator (mass perunit area=60 g/m², thickness=0.13 mm) of the present invention.

Composition of the graft-polymerizing liquid (1) acrylic acid monomer:25 mass % (2) benzophenone: 0.3 mass % (3) iron sulfate: 0.4 mass % (4)nonionic surface active agent: 3 mass % (5) water: 71.3 mass %

Evaluation of Properties

(1) Maximum Pore Diameter

The maximum pore diameter of each of the separators of the presentinvention prepared in Examples 8 and 9 was measured in accordance with abubble point method using a porometer Coulter Electronics Ltd. Theresults are shown in Table 2.

(2) 5% Modulus Strength

Each of the separators of the present invention prepared in Examples 8and 9 was cut to a 50 mm width, and set between chucks (distance betweenthe chucks=10 cm) of a tensile tester (TENSILON UTM-III-100;manufactured by ORIENTEC, Co.), and a force required to extend theseparator by 5 mm (pulling rate=300 mm/min) was measured. The resultsare shown in Table 2.

(3) Ratio of Pores Having a Pore Diameter of 30 μm or Less With Respectto the Whole Pores

For each of the separators of the present invention prepared in Example8 and Example 9, the ratio of pores having a pore diameter of 30 μm orless with respect to the whole of pores was calculated by measuring apressure of gas (nitrogen gas) in accordance with a bubble point method,calculating a pore diameter in accordance with a Washburn equation, andobtaining a distribution of the pore diameter from a flow of the gas.The results are shown in Table 2.

(4) Air Permeability

The air permeability of each of the separators of the present inventionprepared in Examples 8 and 9 was measured in accordance with the methoddefined in JIS L 1096 (1999) [8.27.1 A method (Frazir method)]. Theresults are shown in Table 2.

(5) Electrical Resistance

For each of the separators of the present invention prepared in Examples8 and 9, the “electrical resistance” was determined in accordance withthe procedures as mentioned below, using a test equipment disclosed inJIS C2313 (Separator for a lead secondary battery), 7.2.4 ElectricalResistance, 3. Test Equipment. Specifically, a potassium hydroxidesolution having a specific gravity of 1.3 (measured at 20° C.) wasintroduced into the test equipment, i.e., a test battery container. Aresistance R was determined by passing a direct current of 1A betweenthe current electrodes while the temperature was maintained at 25±0.5°C. in a constant-temperature bath, and measuring a voltage drop due to aliquid resistance by a voltage indicator. Then, three specimens cut fromthe separator to be examined were placed in a position where a specimenis placed, a resistance R₁ was determined by measuring a voltage dropaccording to the above-mentioned method. An electrical resistance R₀ wascalculated from the equation (I):R₀=(R₁−R)/(5×3)  (I)wherein R₀ is an electrical resistance (unit=mΩ·100 cm²/sheet) of theseparator to be examined, R₁ is a resistance (unit=mΩ) obtained when theseparator to be examined was placed, and R is a resistance (unit=mΩ)obtained when the separator to be examined was not placed.

The specimen used was cut from approximately central portion of theseparator to be examined, and had a size of about 70×70 mm. The specimenwas dipped in a potassium hydroxide solution (specific gravity=1.3 at20° C.) at 25±2° C. for 5 hours before the above test procedures. When aseparator to be examined was too small to prepare a specimen of about70×70 mm, a specimen was prepared from the original separator at a rateof one specimen per about 400 cm². As the current electrode in the testequipment, a nickel plate (length=70 mm, width=70 mm, thickness=1 mm)composed of one or more kinds of nickel metals defined in JIS H2105(nickel metal) was used. The voltage electrode used was prepared bydipping a cadmium bar (diameter=about 5 mm, length=about 50 mm or more)composed of one kind of a cadmium metal defined in JIS H 2113 inpotassium hydroxide (specific gravity=1.3 at 20° C.) at an ordinarytemperature for 24 hours or more. The test battery container used was analkali-resistant vessel. When a space was generated upon which thespecimens were placed, the measurement was carried out after thespecimens were fixed by an alkali-resistant fixing spacer having a shapethe same as that of the position where a specimen is placed. In thiscase, the resistance in the absence of the separator to be examined wasmeasured, while the alkali-resistant fixing spacer was inserted.

The results are shown in Table 2.

(6) Ratio of Maintaining Thickness and Ratio of Non-conforming BatteriesProduced During the Battery Assembly

The separators of the present invention prepared in Examples 8 and 9were examined for a ratio of maintaining thickness and a ratio ofnon-conforming batteries produced during the battery assembly inaccordance with the methods as described above, respectively. Theresults are shown in Table 2.

TABLE 2 C F G H J K L Example 8 105 0.1 43 90 98 15 0.0015 Example 9 1020.5 45 74 97 25 0.0010[In Table 2, “C” denotes a ratio of maintaining a thickness (unit=%),“F” denotes a ratio of non-conforming batteries produced during thebattery assembly (unit=%), “G” denotes a maximum pore size (unit=μm),“H” denotes a 5% modulus strength (unit=N/5 cm width), “J” denotes aratio of pores having a pore size of 30 μm or less in the whole pores(unit=%), “K” denotes an air permeability (unit=cm/sec), and “L” denotesan electrical resistance (unit=mΩ·100 cm²/sheet)]

INDUSTRIAL APPLICABILITY

In the alkaline battery separator of the present invention, a shortcircuit caused by a puncturing of the separator due to the occurrence ofa flash on the electrode is avoided, and the separator is prevented frombeing torn by an edge of an electrode, and further a good workability isprovided, and the separator is not wrinkled. Therefore, a battery can bestably produced in a good yield. The alkaline battery separator of thepresent invention has an excellent property required to diffuse a pouredelectrolyte.

As above, the present invention was explained with reference toparticular embodiments, but modifications and improvements obvious tothose skilled in the art are included in the scope of the presentinvention.

1. An alkaline battery separator comprising a fiber sheet comprisingmore than 75 mass percentage of hydrophilicity-imparted polyolefinfibers having a fiber diameter of 9 μm or more, based upon 100% totalweight of constituent fibers of the fiber sheet, wherein a part of saidhydrophilicity-imparted polyolefin fibers is comprised of fibers havinga tensile strength of at least 5 g/d.
 2. The alkaline battery separatoraccording to claim 1, wherein said fiber sheet contains fibers having atensile strength of 5 g/d or more and fusible fibers, as saidhydrophilicity-imparted polyolefin fibers having a fiber diameter of atleast 9 μm.
 3. The alkaline battery separator according to claim 2,wherein the mass ratio of non-fusible fibers having a tensile strengthof at least 5 g/d and said fusible fibers is 10:90 to 50:50.
 4. Thealkaline battery separator according to claim 1, wherein the maximumpore diameter of said fiber sheet is less than 50 μm.
 5. The alkalinebattery separator according to claim 1, wherein a 5% modulus strengthwith respect to at least a direction of said fiber sheet is at least 60N/5 cm width.
 6. The alkaline battery separator according to claim 1,wherein pores having a pore diameter of at least 30 μm or less in saidfiber sheet accounts for at least 95% of the whole of pores.
 7. Thealkaline battery separator according to claim 1, wherein an airpermeability of said fiber sheet is at least 4 cm/sec.
 8. The alkalinebattery separator according to claim 1, wherein an electrical resistanceof said fiber sheet is less than 5 mΩ·100 cm²/sheet.
 9. The alkalinebattery separator according to claim 1, wherein polyethylene fiberswhose surface consists substantially of a polyethylene resin account forat least 60 mass % of said hydrophilicity-imparted polyolefin fiberswhich are the major component of said fiber sheet.
 10. The alkalinebattery separator according to claim 1, wherein said fiber sheet is anon-woven fabric.
 11. A process for producing an alkaline batteryseparator comprising a fiber sheet comprising more than 75 masspercentage of hydrophilicity-imparted polyolefin fibers having a fiberdiameter of at least 9 μm, comprising steps of: forming a fiber sheetfrom polyolefin fibers which have a fiber diameter of at least 9 μm,said polyolefin fibers containing polyolefin fibers having a fiberdiameter of at least 9 μm and a tensile strength of at least 5 g/d; andthen imparting a hydrophilic property to the resulting fiber sheet.